14 ZOOLOGY. 



point where its presence is required. The saliva, tears, and 

 perspiration, are examples of the secretions of glands. 



4. The Teeth. The mouth is bounded by the upper 

 and lower jaws, bearing the teeth. In front are the chisel- 

 like incisors a double pair above, and single pair below. 

 These nibble off pieces of roots, seeds, etc., which are then 

 crushed and ground by the cheek-teeth (three premolars and 

 three molars above, two premolars and three molars below), 

 which have flat crowns crossed by harder ridges. Between 

 the incisors and cheek-teeth there is a gap : in carnivorous 

 animals like the dog this space is filled by the biting canines, 

 and the cheek-teeth are cutting, not grinding, structures. 



The student will probably ask why the cheek-teeth, 

 which are all similar in appearance, are divided into pre- 

 molars and molars. The young rabbit has a set of milk 

 molars a milk dentition which is followed by the per- 

 manent teeth, just as in man. Those cheek-teeth of the 

 second set, which have predecessors in the first series, are 

 called premolars ; the ones posterior to these are the molars. 



5. The Saliva. When the material is well ground up, 

 it mixes with the saliva, a fluid secreted in a series of 

 glands about the face and throat (submaxillary, sublingual, 

 infra-orbital, and parotid glands). This fluid consists of 

 water, with a certain amount of mucin (the slimy compound 

 familiar to us in the secretion of, the nose), various salts in 

 small proportions, and (most important of all) a ferment 

 called ptyalin. This last has the effect of changing the in- 

 soluble starch into sugar, which at once begins to dissolve 

 in the water. The mucin lubricates the crushed-up food, 

 and so makes swallowing easier. The saliva is of great 

 importance in all plant-eating land-animals, as their food 

 contains much starch. In such an animal as the dog, 

 which has but little starch in its food, there is very little 

 chewing (mastication) and the saliva is of less importance. 



6. The Tongue. On the floor of the mouth is the 

 tongue, which we shall refer to again in chapter x., in con- 

 nexion with the sense of taste. Here we may note that it 



